This invention relates generally to an epoxy resin composition and, more specifically, to an improved epoxy resin composition suitable for bonding electric parts onto printed wiring boards using a dispenser.
It is known to use an epoxy resin adhesive composition for bonding electric parts such as IC elements to printed wiring boards. In this case, it is the general practice to apply the adhesive composition to the board by means of a dispenser.
One known epoxy resin composition suited for dispensation with a dispenser includes an epoxy resin, an amine-type curing agent, a thixotropic agent and hydrophilic silica (Japanese Published Unexamined Patent Application No. 4-33916). The known composition shows little change in viscosity with time. Further, the shape of a dot of the composition applied on the board almost remains unchanged during the course of curing. However, the known composition has been found to be ill-suited for used with a high speed dispenser operated with a tact time of, for example, below 0.15 second. In particular, when a series of dots of the adhesive composition are successively formed on a board at a high speed with a dispenser, a greater part of the dots thread in the direction parallel with the moving direction of the dispenser relative to the board. Namely, as shown in FIGS. 1(a) and 1(b), a dot 2 applied onto a board 1 threads to form a tail-like portion 3 along the movement of a dispensing nozzle 4 in the direction shown by the arrow relative to the board 1. When the length of the tail-like portion 3 exceeds 20% of the diameter of the dot 2, it is necessary to slow down the tact time of the dispenser.